fix error: std::coroutine_traits isn't a class template

[sizeofvoid]

Simple fix to prevent a build error with LLVM16:

FAILED: examples/basics/CMakeFiles/await-sync-string.dir/await-sync-string.cpp.o 
/usr/ports/pobj/qcoro-0.9.0/bin/c++ -DQT_NO_CAST_FROM_ASCII -DQT_NO_CAST_FROM_BYTEARRAY -DQT_NO_CAST_TO_ASCII -DQT_NO_FOREACH -DQT_NO_KEYWORDS -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_NO_URL_CAST_FROM_STRING -DQT_STRICT_ITERATORS -DQT_USE_STRINGBUILDER -I/usr/ports/pobj/qcoro-0.9.0/build-amd64/examples/basics/await-sync-string_autogen/include -I/usr/ports/pobj/qcoro-0.9.0/qcoro-0.9.0 -I/usr/ports/pobj/qcoro-0.9.0/qcoro-0.9.0/qcoro -I/usr/ports/pobj/qcoro-0.9.0/build-amd64/qcoro -O2 -pipe -DNDEBUG -std=gnu++20 -MD -MT examples/basics/CMakeFiles/await-sync-string.dir/await-sync-string.cpp.o -MF examples/basics/CMakeFiles/await-sync-string.dir/await-sync-string.cpp.o.d -o examples/basics/CMakeFiles/await-sync-string.dir/await-sync-string.cpp.o -c /usr/ports/pobj/qcoro-0.9.0/qcoro-0.9.0/examples/basics/await-sync-string.cpp
In file included from /usr/ports/pobj/qcoro-0.9.0/qcoro-0.9.0/examples/basics/await-sync-string.cpp:5:
/usr/ports/pobj/qcoro-0.9.0/qcoro-0.9.0/qcoro/coroutine.h:97:1: error: std::coroutine_traits isn't a class template
using coroutine_traits = std::experimental::coroutine_traits<R, ArgTypes ...>;
^

$ clang++ --version OpenBSD clang version 16.0.6 Target: amd64-unknown-openbsd7.4 Thread model: posix InstalledDir: /usr/bin

[danvratil]

Thanks for the fix!